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We present Keck NIRSPEC and Keck NIRES spectroscopy of sixteen metal-poorgalaxies that have pre-existing optical observations. The near-infrared (NIR)spectroscopy specifically targets the HeI 10830A emission line, due to itssensitivity to the physical conditions of the gas in HII regions. We use theseNIR observations, combined with optical spectroscopy, to determine the heliumabundance of sixteen galaxies across a metallicity range 12 + log (O/H) = 7.13- 8.00. This data set is combined with two other samples where metallicity andhelium abundance measurements can be secured: star-forming galaxies selectedfrom the Sloan Digital Sky Survey spectroscopic database and existinglow-metallicity systems in the literature. We calculate a linear fit to thesemeasurements, accounting for intrinsic scatter, and report a new determinationof the primordial helium number abundance, y_P = 0.0805+/-0.0017, whichcorresponds to a primordial helium mass fraction Y_P = 0.2436+/-0.0040. Usingour determination of the primordial helium abundance in combination with thelatest primordial deuterium measurement, (D/H)_P x 10^5 =2.527+/-0.030, weplace a bound on the baryon density Omega_bh^2 = 0.0215+/-0.0005 and theeffective number of neutrino species Neff = 2.85+/-0.28. These values are in1.3-sigma agreement with those deduced from the Planck satellite observationsof the temperature fluctuations imprinted on the Cosmic Microwave Background.

The PHLEK Survey: A New Determination of the Primordial Helium Abundance